The conversion of tropical forests to rubber plantations accelerates soil acidification and changes the distribution of soil metal ions in topsoil layers

Unprecedented economic growth in Southeast Asia has encouraged the expansion of rubber plantations. This study aimed to clarify the effects of the conversion of tropical forests to rubber plantations on soil acidification processes, exchangeable cations, exchangeable aluminum (Al), available copper (Cu), zinc (Zn), manganese (Mn) and iron (Fe). The results showed that these conversions significantly decreased soil pH, exchangeable Ca, Mg, available Zn, Mn and Fe, and increased exchangeable Al and available Cu in the topsoil layers. In both the rainy and dry seasons, the conversion of tropical forests to mature rubber plantations increased the average soil exchangeable Al by 930.1 and 54.4%, and soil available Cu by 82.7 and 65.8%, and decreased soil pH by 13.4 and 9.9%, soil exchangeable Ca by 70.9 and 79.9%, soil exchangeable Mg by 76.5 and 77.8%, soil available Zn by 73.8 and 51.6%, soil available Mn by 33.1 and 47.5% and soil available Fe by 15.9 and 22.2% in the 0–10 and 10–30 cm soil layers, respectively. The change of soil exchangeable Al was greatly affected by soil acidification processes and soil organic carbon, exchangeable Ca, Mg and available Cu was greatly affected by soil acidification processes, as were available Zn, Mn, and Fe by soil organic carbon. The large losses of soil exchangeable Ca, Mg, and available Zn in the rubber plantations limited plant growth. The release of large amounts of exchangeable Al in the rubber plantations not only decreased soil available P but also threatened the safety of the surrounding environment.